On July 5, 2013, at about 10:50 p.m., an MMA train carrying petroleum crude oil in 72 Class 111 tank cars arrived at Nantes, Quebec. In keeping with the railway’s practice, the locomotive engineer parked the train for the night on a descending grade on the main track.

After shutting down four of the five locomotives, the engineer applied seven hand brakes. Railway rules require that hand brakes alone must be capable of holding a train, and this must be verified by a test. That night, however, the locomotive air brakes were left on during the test, meaning the train was being held by a combination of hand brakes and air brakes. It also gave the false impression that the hand brakes alone were enough.

The engineer then contacted two rail traffic controllers: one in Farnham, Quebec, to let him know the train was secure; and the other in Bangor, Maine, to discuss the smoking lead locomotive and the problems it might cause for the next crew. As the smoke was expected to settle, it was agreed to leave the train as it was and deal with it the next morning.

Shortly after the engineer left for the night, the Nantes Fire Department responded to a 911 call of a fire on the train. Firefighters extinguished the blaze on the lead locomotive by shutting off the fuel. Then, following railway instructions, they turned off the electrical breakers. After discussing the situation with the rail traffic controller in Farnham and an MMA employee who had been dispatched to the scene, everyone departed.

With all the locomotives shut down, the air brake system began to lose pressure, and the brakes gradually became less and less effective. About an hour later, just before 1 am, the air pressure dropped to a point where the combination of air brakes and hand brakes could no longer hold the train. The train then began to roll downhill toward Lac-Mégantic, seven miles away.

As it moved down the grade, the train picked up speed, reaching a top speed of 65 miles per hour. The train derailed just past the Frontenac Street crossing.

Almost all of the derailed tank cars were damaged, many of them from large breaches, and about 6 million litres of crude oil began pouring into the streets. The fire began almost immediately, and the ensuing blaze and explosions left 47 people dead. Another 2000 citizens were forced from their homes, and much of the downtown core was destroyed.